spi: tegra210-quad: Protect curr_xfer in tegra_qspi_combined_seq_xfer

The curr_xfer field is read by the IRQ handler without holding the lock
to check if a transfer is in progress. When clearing curr_xfer in the
combined sequence transfer loop, protect it with the spinlock to prevent
a race with the interrupt handler.

Protect the curr_xfer clearing at the exit path of
tegra_qspi_combined_seq_xfer() with the spinlock to prevent a race
with the interrupt handler that reads this field.

Without this protection, the IRQ handler could read a partially updated
curr_xfer value, leading to NULL pointer dereference or use-after-free.

Fixes: b4e002d8a7ce ("spi: tegra210-quad: Fix timeout handling")
Signed-off-by: Breno Leitao <[e-mail]>
Tested-by: Jon Hunter <[e-mail]>
Acked-by: Jon Hunter <[e-mail]>
Acked-by: Thierry Reding <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi-tegra210-quad.c

···
 	u32 address_value = 0;
 	u32 cmd_config = 0, addr_config = 0;
 	u8 cmd_value = 0, val = 0;
+	unsigned long flags;
 
 	/* Enable Combined sequence mode */
 	val = tegra_qspi_readl(tqspi, QSPI_GLOBAL_CONFIG);
···
 			tegra_qspi_transfer_end(spi);
 			spi_transfer_delay_exec(xfer);
 		}
+		spin_lock_irqsave(&tqspi->lock, flags);
 		tqspi->curr_xfer = NULL;
+		spin_unlock_irqrestore(&tqspi->lock, flags);
 		transfer_phase++;
 	}
 	ret = 0;
 
 exit:
+	spin_lock_irqsave(&tqspi->lock, flags);
 	tqspi->curr_xfer = NULL;
+	spin_unlock_irqrestore(&tqspi->lock, flags);
 	msg->status = ret;
 
 	return ret;